The invention concerns an object made of aluminum or aluminum alloys in a form such as sheet, foil, containers and the like and having an anodically oxidized surface layer. The said object is suitable for use in printing by means of sublimation thermal printing.
Aluminum and aluminum based alloys in the form of finished or semi-finished products, which are expected to exhibit good corrosion resistance and wear resistance as well as having an attractive, decorative appearance, are usually given an anodic oxidation treatment.
An oxide layer is formed in electrolytes which are generally made up of dilute sulphuric acid, sometimes with additions of oxalic acid, less often of dilute oxalic acid alone, or of dilute phosphoric or chromic acid, and by applying an electrical current, principally in the form of direct current, less often in the form of alternating current or by superimposing or switching alternating and direct current, the items or semi-fabricated product being made the anode in the circuit.
These oxide layers are in general made up of a very thin, almost pore-free dielectric base layer, the so called barrier layer, and on top of this a top layer which has many fine pores in it. The barrier layer is self-generating, being formed by the conversion of aluminum to aluminum oxide and this at the same rate as the top layer is formed during anodic oxidation.
The top layer is made up of bundles of fibers which lie essentially perpendicular to the surface of the metal and in general are transparent and colorless when produced using dilute sulphuric acid as the electrolyte and direct current.
There are many processes which can be used to produce color effects in the anodic oxide layer on aluminum. These processes can be divided into four groups according to the way they work:
(1) Color can be introduced by using special electrolytes, e.g., aqueous solutions of carbonic acid or sulphonic acid.
(2) Deposition of metals in the pores in the fiber bundle of the top layer of a transparent, colorless oxide layer, by means of an alternating current applied in an aqueous metal salt solution.
(3) Deposition of inorganic pigments or organic coloring agents in the pores in the fiber bundle of the top layer of a transparent, colorless anodic layer by means of immersion in a warm solution containing the coloring substance.
(4) Deposition of organic coloring agents in the pores in the fiber bundle making up the top layer of a transparent, colorless oxide layer by bringing the oxide into direct contact with a hydrolysis-resistant, coloring agent which can sublimate and which is printed on a substrate, e.g., a paper substrate, with the result that the anodic oxide layer sucks up the coloring agent into the pores in the fiber bundle under the influence of heat. The coloring agents which are suitable for this process are dispersion coloring substances with anthrachinon as the basis with at least one of the positions 1, 4, 5 or 8 occupied by either H, OH--, amino or amido groups and at least one active hydrogen, or azo coloring agents with an OH-- group in the ortho position of the azo group, or coloring agents with a 1,3-indandion group.
After the coloring substance has been deposited in the oxide, the pores in the anodic oxide layer containing the coloring substance are closed or sealed, as by a treatment in hot, deionized water. As a result of the hot water treatment, at least a part of the Al.sub.2 O.sub.3 of the newly produced oxide layer is converted to AlOOH, so called pseudo-boehmite.
On looking at the four different processes for coloring anodic oxide layers on aluminum, it is clear that anodic oxide layers which are multi-colored, patterned or carrying a picture can be produced commercially in a particularly favorable manner by the process listed under point (4), viz., using the transfer of coloring material which can be sublimated from a paper substrate under the influence of contact pressure and heat by so called heat-transfer printing.
This process which has been known for some time now has not been able to develop into a usable technology as it suffered from the serious disadvantage that on transferring the hydrolysis-resistant, sublimable organic coloring substance from the substrate to the absorbant 5 to 20 .mu.m thick anodic oxide layer by heating to the temperature of 120.degree. to 220.degree. C. necessary for that process, fine hair-line cracks occured and these were disturbing to the eye especially when viewed at acute angles of incident light.
The object of the present invention, therefore, is to develop an object made of aluminum or aluminum alloys which has an oxide produced by anodic oxidation which can be given a colored image using sublimable, hydrolysis-resistant coloring substances by means of heat-transfer printing without the oxide layer afterwards exhibiting disturbing hair-line cracks due to the effect of the temperature required for the sublimation process.